In Long Term Evolution (LTE for short), a physical downlink control channel (PDCCH for short) is used for bearing uplink and downlink scheduling information and uplink power control information. The downlink control information (DCI for short) formats includes DCI formats 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 3, 3A, etc. And later DCI formats 2B, 2C, and 2D are added to an evolved LTE-A Release 12 to support a variety of different applications and transmission modes. A base station (e-Node-B, eNB for short) may configure a user equipment (UE for short) through the downlink control information, or the UE is configured by the high-layer, which is also referred to as being configured with the high-layer signaling.
A sounding reference signal (SRS for short) is a signal used between the UE and the eNB for measuring radio channel state information (CSI for short). In the LTE system, the UE periodically transmits an uplink SRS on the last data symbol of a transmission subframe according to parameters, indicated by the eNB, such as a frequency band, a frequency domain position, a sequence cyclic shift, a period, and a subframe offset. The eNB determines the uplink CSI of the UE according to the received SRS, and performs operations such as frequency domain selection scheduling, closed loop power control according to the obtained CSI.
In a study of LTE-A Release 10 (LTE-A Release 10), it is proposed that in uplink communication, a non-precoded SRS, that is, an antenna-specific SRS should be used, while a demodulation reference signal (DMRS for short) used for demodulation in a physical uplink shared channel (PUSCH for short) is precoded. The eNB can estimate original uplink CSI by receiving the non-precoded SRS, while can not estimate the original uplink CSI through the precoded DMRS. At this time, when the UE transmits the non-precoded SRS by using multiple antennas, more SRS resources are required by each UE, which results in a decrease in the number of UEs that can be simultaneously reused in the system. The UE can transmit the SRS in two triggering manners, that is, through the high-layer signaling (also referred to as the trigger type 0) or the downlink control information (also referred to as the trigger type 1). A periodic SRS is triggered based on the high-layer signaling, and a non-periodic SRS is triggered based on the downlink control information. In LTE-A Release 10, a manner of a non-periodic transmission of SRS is added, which improves the utilization rate of SRS resources to some extent and improves the flexibility of resource scheduling.
With the development of communication technologies, the demand for data services is increasing and available low-frequency carriers are in short supply. Therefore, high-frequency (30 GHz to 300 GHz) carrier communication which has not been fully utilized has become an important communication way of achieving high-speed data communication in the future. The high-frequency carrier communication has a large available bandwidth and can provide effective high-speed data communication. However, a big technical challenge for the high-frequency carrier communication is that high-frequency signals are attenuated significantly in space compared with low-frequency signals. Although this will cause spatial attenuation losses when the high-frequency signals are used for outdoor communication, the shorter wavelength of the high-frequency signals usually allows using more antennas. Therefore, the communication is implemented based on beams to compensate the spatial attenuation losses.
However, when the number of antennas increases, each antenna needs a set of radio frequency links, and digital beamforming thus brings about an increase in costs and a loss in power. Therefore, current studies tend to hybrid beamforming, that is, a final beam formed by radio frequency beams together with digital beams.
In a study of the new radio access technology (NR for short), for the high-frequency communication system, the eNB is configured with a large number of antennas to form downlink transmission beams for compensating the spatial attenuation of the high-frequency communication, and the UE is also configured with a large number of antennas to form uplink transmission beams. At this time, the SRS is also transmitted in the form of a beam. In a future study of the new radio access technology, the eNB may configure different bandwidth parts (BWP for short) for each user, and the bandwidth occupied by the BWP of a user may be larger than the 20 MHz bandwidth of the LTE or LTE-A system. The current SRS bandwidth configuration only supports 20 MHz at most, which cannot meet the design requirements of NR. In addition, how to determine a frequency domain starting position of the SRS and how to achieve an antenna switching of the SRS are also problems to be solved in the SRS design of NR.